Different kinds of oscillator circuits, i.e. oscillators, are used for a very great number of applications of the electronics and the telecommunications technology. Typical applications of the telecommunications technology are Phase-Locked Loops (PLL), frequency oscillators, modulators, etc.
Oscillator circuits, i.e. oscillators, can be implemented by many different circuit structures. One of them is an astable (free-running) multivibrator. FIG. 1 shows a conventional emitter-coupled multivibrator circuit. The circuit comprises two transistors Q1 and Q2, between which is provided a positive feedback by connecting each transistor base to the collector of the other transistor. In some known solutions, Rc1 and Rc2 are replaced by coils. The collectors of Q1 and Q2 are connected via the resistors Rc1 and Rc2, respectively, to one potential of an operating voltage source 1 and the emitters are connected via current sources 3 and 4, respectively, to the lower potential of the operating voltage source. Additionally, a capacitance C is connected between the emitters of Q1 and Q2. The positive feedback and series resonance circuits Rc1-C and Rc2-C constituted by the resistors RC1 and RC2 and the capacitance C lead to that the multivibrator output oscillates continuously be between two states, after the oscillation once has been trigged. The oscillation frequency is determined by component values of the RC series resonance circuits. The oscillation frequency can be controlled by changing some of these component values, typically the capacitance C.
In the following, the operation of the multivibrator will be examined closer. To begin with, it is assumed that Q1 is off (non-conduction state). When Q1 is off, the collector of Q1 and the base of Q2 are generally at the operating voltage potential. Then Q2 is on (conducting state) and its emitter current is I1+I2. In other words, when Q2 is conductive, the current I1 flows from the emitter of Q2 via the capacitance C to the emitter of Q1. Then the current I1 charges/discharges the charge of the capacitance C, whereby the emitter potential of Q1 falls at a predetermined speed until Q1 becomes conductive when the base emitter voltage of Q1 exceeds appr. 0.6 V. When Q1 becomes conductive, its collector voltage begins to fall. On account of positive feedback, the base voltage of Q2 falls as well and Q2 closes. When Q2 is off and Q1 is on, the current I2 flows from the emitter of Q1 via the capacitance C to the emitter of Q2, where the emitter voltage begins to fall until it makes Q2 open and Q1 close again. The speed of such a multivibrator circuit (maximum resonance frequency) depends primarily on the properties of the transistors Q1 and Q2. One known way of increasing the speed of the multivibrator circuit is to provide a positive feedback from the collector of one transistor to the base of the other transistor via a buffer transistor. This enables a higher base current, which again discharges parasitic capacitances of the base circuit of the transistor faster and accelerates thus the switching of the transistor from one state to another. Nowadays, there is a need of ever-increasing speeds, however.
The minimum operating voltage of a multivibrator of above type is about 1.5 V, from which at least 0.4 to 0.5 V is used across the current sources 3 and 4. Especially in electronic equipments using battery power supplies, operating voltages lower than this would still be desired. In a buffered multivibrator circuit, the operating voltages are higher than this, however.